[Haskell-cafe] How to optimize a directory scanning?

[Magicloud Magiclouds]

I could not tell, since those are some kind of "standard" functions of
Haskell, right?

On Fri, May 10, 2019 at 10:11 AM David Feuer <david.feuer at gmail.com> wrote:
>
> Pure speculation: are you paying for a lot of conversions between FilePath (string) and C strings?
>
> On Thu, May 9, 2019, 10:02 PM Magicloud Magiclouds <magicloud.magiclouds at gmail.com> wrote:
>>
>> Hi,
>> I have asked this in Stackoverflow without getting an answer.
>> Wondering if people here could have some thoughts.
>>
>> I have a function reading the content of /proc every second.
>> Surprisingly, its CPU usage in top is around 5%, peak at 8%. But same
>> logic in C or Rust just takes like 1% or 2%. Wondering if this can be
>> improved. /proc is virtual filesystem, so this is not related to HDD
>> performance. And I noticed this difference because my CPU is too old
>> (Core Gen2). On modern CPU, as tested by others, the difference is
>> barely noticeable.
>>
>> import Control.Exception
>> import Control.Concurrent
>> import Control.Monad
>> import Data.Char
>> import Data.Maybe
>> import System.Directory
>> import System.FilePath
>> import System.Posix.Files
>> import System.Posix.Signals
>> import System.Posix.Types
>> import System.Posix.User
>> import System.IO.Strict as Strict
>>
>> watch u limit0s limit0h = do
>>   listDirectory "/proc/" >>= mapM_ (\fp -> do
>>     isMyPid' <- maybe False id <$> wrap2Maybe (isMyPid fp u)
>>     wrap2Maybe (Strict.readFile ("/proc/" </> fp </> "stat")))
>>   threadDelay 1000000
>>   watch u limit0s limit0h
>>   where
>>     wrap2Maybe :: IO a -> IO (Maybe a)
>>     wrap2Maybe f = catch ((<$>) Just $! f) (\(_ :: IOException) ->
>> return Nothing)
>>     isMyPid :: FilePath -> UserID -> IO Bool
>>     isMyPid fp me = do
>>       let areDigit = fp >= "0" && fp <= "9"
>>       isDir <- doesDirectoryExist $ "/proc/" </> fp
>>       owner <- fileOwner <$> getFileStatus ("/proc" </> fp)
>>       return $ areDigit && isDir && (owner == me)
>>
>>
>> --
>> 竹密岂妨流水过
>> 山高哪阻野云飞
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-- 
竹密岂妨流水过
山高哪阻野云飞

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